DAQmx Measures Values Above or Below the Set Maximum or Minimum

Issue Details

I have set a Max and Min input value for my DAQ device channel and when I start acquiring data, it is displaying values above the maximum and below the minimum. Why does it not restrict measurements to the maximum and minimum that I set?

Solution

In both the NI MAX Test Panels as well as the DAQmx Create Virtual Channel VI, there are inputs that allow the user to specify a maximum and minimum value that is expected to be measured on that channel. This value is used only to select the internal ADC amplifier to be used for that channel, and does not restrict input values to the input max/min values. This means that, although you may specify a maximum and minimum value, if the input signal being sampled is below the minimum or above the maximum, the DAQ channel will still read the accurate value of the signal. This is important to ensure that signal integrity is maintained and the accuracy of the measurements remains constant. 

This does not mean that signals will not be clipped, or truncated. When the ADC amplifier is selected by the DAQmx driver based on the user max/min inputs, any signal outside the bounds of the selected amplifier range will be clipped. 

For example, if you have the following configuration:

Hardware: NI-9205 C Series Voltage Input Module
User-set maximum input voltage: +3V
User-set minimum input voltage: -3V

The values set as the max and min input voltages would dictate that the ADC amplifier in use allows for a ±5V signal. If the input signal reaches +4V, the device will still measure and report a signal value of +4V because, although the user set the maximum to +3V, the +4V value is still within the limits of the ADC amplifier. If the input signal reached +5.5V, however, the device will measure +5V because the +5.5V signal is outside the limits of the ADC amplifier.